1. Field of the Invention
The present invention relates generally to electrically propelled wheelchairs. More particularly, the present invention relates to apparatus and method for selectively adjusting both maximum speed and maximum torque of first and second motor drives that propel and steer electrically powered wheelchairs.
2. Description of the Related Art
Electrically propelled wheelchairs provide mobility to those who cannot walk, thereby helping them to partially overcome their physical disabilities. This makes the physically handicapped more productive, and it gives them a greater sense of purpose and satisfaction with life.
Having a suitable power wheelchair is especially important for children who cannot walk and will never be able to walk, and who are unable to use a manually-propelled wheelchair. When provided with power wheelchairs that they can operate satisfactorily, small children blossom in their personality development, in their mental development, and in their total outlook of life.
As can be easily understood, their power wheelchair becomes an integral part of their life, so integral in fact, that it becomes almost a part of them. With it, they have the ability to move from place to place. They are a person with the ability to enter in and be a part of activities surrounding them. Without it, they are doomed to be placed on the floor or on a chair, feeling more like a piece of the furniture than a person.
This importance of a power wheelchair to a child places stringent requirements on reliability, maintenance costs, low weight, and ease of control.
Power wheelchairs must, as nearly as possible, be maintenance free. Obviously, even limited times without the use of his power wheelchair seems to be an unbearably long time to a child. Further, if a child has never walked, and will never walk, this mobility must not be taken away from him when he is outdoors. He must be able to use his wheelchair outdoors, and he will be caught out in rain showers from time to time. Therefore, the electrical and electronic system of his power wheelchair must be impervious to sun, rain, dust, and mud.
Parents of handicapped children are typically made poor by the medical expenses that they have occurred in the past, and/or they are kept poor by continuing medical costs. While help is usually available for the original purchase of power wheelchairs for those needing them, costs to repair poorly designed power wheelchairs can be too expensive for parents to keep their child's power wheelchair operating.
Weight of the power wheelchair must be as low as possible, thus system efficiency must be high, so that battery weight can be low. Further, it must be possible to remove and replace the battery in a few seconds. A van with a lift can transport a heavy power wheelchair, but for a wheelchair to meet a child's needs, it must be light enough to be carried up the stairs of his grandmother's house.
Typically, an electrically powered wheelchair is controlled by a manually-actuated X-Y controller. That is, the power wheelchair is controlled by moving a control handle forward from a neutral position for forward propulsion, rearward for rearward propulsion, forward and to the right for a right turn, and to the right for a pivot turn in which one wheel rotates in one direction and the other wheel rotates in the other direction. In all of these maneuvers, the speed of movement should be proportional to movement of the control handle from the neutral position.
Other types of controls have been used, and are used, for children, adults, and older adults who do not have the strength or manual dexterity to operate this type of manual control. However, children who are able to use an X-Y controller, and their parents, obtain great satisfaction from this accomplishment.
Typical prior art X-Y controllers have utilized two electrical potentiometers disposed at 90 degrees to each other and disposed at 45 degrees to X and Y axes. Because of this geometry, movement of the control handle forward along the Y axis to a maximum forward propulsion position has produced rotation of both potentiometers that is a function of the sine of 45 degrees, thereby producing only about 70 percent of the maximum output of both potentiometers and about 70 percent of the maximum speed of both motors.
However, in response to maximum movement of the control handle in a direction that is forward and 45 degrees to one side of the Y axis, this movement of the control handle has produced an output of one potentiometer and a speed in one motor that is a function of the sine of zero degrees, or zero. This same control handle movement has produced an output of the other potentiometer and a speed in the other motor that is a function of the sine of 90 degrees, thereby producing a speed of 100 percent of maximum.
Therefore, when attempting to negotiate turns, this type of X-Y controller has caused the wheel on the outside of the turn to increase in speed by 41 percent, making turns difficult to control. This same characteristic of this type of X-Y controller has made power wheelchairs unduly sensitive to turns, making it difficult to even steer the wheelchair in a straight line.
This control problem was overcome by an X-Y controller shown and described in U.S. patent application Ser. No. 07/590,876 by the inventor of the present invention. This X-Y controller never produces, in response to any angle of movement of the control handle, an output that is greater than movement of the control handle along the Y axis. In addition, this improved controller produces maximum electrical outputs that are selectively-chosen percentages of X axis inputs.
Finally, especially for power wheelchairs used by children either attending school or having young brothers or sisters, and for power wheelchairs used by adults in nursing homes, both maximum speed and maximum torque of the power wheelchairs must be carefully limited. Otherwise, those using the wheelchair may become a danger to others, whether to children playing on the floor, or to ambulatory geriatric patients.
As is well-known, the speed of D.C. motors is highly load sensitive. Feedback can be used from the electric motors making the electrical drive system entirely dependent upon the selective positioning of the unitary control. However, a system in which speed is entirely unrelated to forces placed upon it by obstacles placed in its way can be dangerous to others and hazardous to various pieces of equipment and furniture, including the power wheelchair.
Further, if no feedback is used, and if the maximum speed of the wheelchair is adjusted on a hard and level surface to a suitable range for pediatric or geriatric use, the wheelchair may not have sufficient power for use on a carpet, on unpaved ground, or up an incline.
Also, if the system has means for reducing the ratio of electrical output to mechanical input when making turns, such as the X-Y controller of U.S. patent application Ser. No. 07/590,876, the effective driving voltage to the motor powering the wheel on the outside of the turn will be reduced when making turns. Thus, when attempting sharp turns, or pivot turns, there may be insufficient power to achieve the desired maneuver, and the power wheelchair may stall.
In addition, it can be understood that, for different children or adults, different maximum speeds and different maximum torque levels are imperative. Also, at different times, and under different conditions, a given child or adult will need his wheelchair set to different limits of speed and torque. Thus, it is important, not only for physical therapists, but also for parents or other family members, to be able to reset the maximum speed and/or maximum torque at any time without special equipment or expertise.
Therefore, while the prior art includes digital designs that are programmable by factory-trained experts, these designs do not meet the criteria for ease of adjustment by health care personnel and family members. Further, while the prior art has adjusted operating parameters by digital means while the wheelchair was stationary, both maximum speed and maximum torque can most easily and most accurately be adjusted while the wheelchair is operating on various surfaces, on various inclines, or in various maneuvers. Further, analog adjustments are much more conveniently made than digital adjustments, and an ideal system includes means for easily and quickly activating and deactivating the ability to make changes in maximum speed and torque.
Prior art digital controls have been used, but they have failed to meet the requirements for pediatric power wheelchairs in that they have been relatively poor in reliability, have been sold with short warranty times, have been excessively high in maintenance costs, and have been prone to failure even when exposed to only moderate rain showers.
Finally, since the current flow of an electric motor is a function of the torque load placed on the electric motor, typically, torque sensing has been achieved by placing a resistor in series with the electric motor. In order to minimize power loss in this load-sensing resistor, a very low resistance, sometimes in the order of 0.01 ohms, has been used. Melocik, in U.S. Pat. No. 4,361,788, and Klimo, in U.S. Pat. No. 4,387,325, are typical of the prior art.
With the load-sensing resistor placed in series with the electric motor, this resistor senses motor current without regard to time or function of the motor. More particularly, if the driving voltage is a pulsed voltage, whether of pulse duration or pulse frequency type, sensing is done at all times, both during times in which the driving voltage pulses are being delivered to the electric motor and intermediate of the driving voltage pulses.
Therefore, this type of sensing, if used with a low load-sensing resistance, produces a very low signal which is subject to corruption by noise voltages. But, if used with a higher load-sensing resistance, the resistance wastes power excessively and decreases the operating time of the battery between charges. Further, since torque sensing of the load resistor continues both during driving pulses and the spaces therebetween, the torque signal, which is already too low, and which must be amplified too much to produce the best results, is contaminated by being mixed with a signal taken during times in which the pulses of driving voltage are not being supplied to the electric motor.